1. Field of the Invention
The present invention relates to an improved positive electrode for a lithium ion secondary battery, and a lithium ion secondary battery using the same.
2. Description of the Related Art
A lithium ion secondary battery has features such as high electromotive force and high energy density, and therefore, demands as a power supply of portable equipment such as personal computers and cellular phones have recently increased. With respect to the battery applied in the above uses, it is required to use at normal temperature and simultaneously to have high energy density and excellent cycle characteristics. A novel active material with high capacity has been developed for a positive electrode and a negative electrode so as to respond to these requirements. In the positive electrode, a lithium nickelate-based material has been studied as a positive electrode active material which enables high capacity.
However, the lithium nickelate-based material has a problem that lithium hydroxide or lithium carbonate is produced as a result of moisture absorption, and thus the surface layer is likely to be deteriorated. In the lithium nickelate-based positive electrode active material in which lithium hydroxide or lithium carbonate is produced on the surface as a result of moisture absorption, gas generation may be caused by decomposition of lithium hydroxide or lithium carbonate in the battery upon the charging and discharging cycle or during storage at high temperature, resulting in deterioration of battery performances.
To cope with these problems, it is proposed to improve battery characteristics such as cycle lifetime and irreversible capacity by using an active material powder coated with a silane coupling agent. For example, Patent Document 1 (Japanese Unexamined Patent Publication (Kokai) No. 08-111243) proposes to decrease irreversible capacity by coating a positive electrode active material powder with a coupling agent, applying the coated positive electrode active material powder on a current collector, and using the resulting product as a positive electrode plate. Also, Patent Document 2 (Japanese Unexamined Patent Publication (Kokai) No. 11-354104) discloses that a lithium ion secondary battery having small irreversible capacity, higher charging and discharging efficiency and more excellent charging and discharging cycle characteristics is obtained by treating a surface of an active material with a coupling agent.
Both Patent Documents 1 and 2 propose that an active material powder is subjected to treatment by a coupling agent and a mixture paste containing the active material powder is prepared, and then the resulting mixture paste is applied on a current collector, followed by pressure bonding and further drying to obtain an electrode plate.
However, according to conventional methods, as proposed in Patent Documents 1 and 2, in which active material powder particles are coated and then, a mixture layer containing the same is processed (rolled) by applying a pressure, the lithium nickelate-based active material easily causes cracks or deformation of active material particles as a result of rolling the electrode plate, and thus an interface free from the coated coupling agent is likely to be produced, which makes it difficult to obtain sufficient effect of suppressing moisture absorption through the coating.